M. Ussow’s Zoologico-Eimbryological Investigations. 329 
Phallusia mammillata). The well-known ramified, claviform, 
cecal tubes*, which are met with in the second and third 
layers of the outer mantle, are developed in the embryos of 
Cynthia and Phallusia from five cecal diverticula growing 
forth from the main ventral artery-vein (Phallusia mammil- 
lata), and afterwards gradually becoming elongated and rami- 
fying dichotomously. 
The whole system of the tubes thus ramified forming two 
rings (Cynthia microcosmus) is nothing but the whole system of 
the capillary blood-vessels of the outer mantle, which is united 
to the heart by a thick branch growing forth from the main 
ventral artery-vein. At the beginning of ramification, the 
thicker blood-vessels have three-layered walls, possessing an 
external fibrous layer, a middle one consisting of muscular- 
fibre rmgs, and an inner one composed of hexagonal epithelial 
cells. By the contractions of the muscular layer of the vessels 
and the pulsations of the heart, the blood is driven to the 
remotest periphery of the outer mantle. ‘The walls of the fine 
capillary vessels and their claviform enlargements consist of a 
simple epithelial layer. ‘The complete circulation of the blood 
in the outer mantle is effected in two ways:—1l. All the 
vessels, including their enlarged parts, consist of two tubes 
grown together ; and in these double vessels, if they may be so 
called, the centrifugal stream of blood moves on one side, and 
at the same time the centripetal one on the other (as may be 
observed, for example, in the embryos of Pyrosomat). 2. 
The whole network of blood-vessels of the outer mantle is 
divided into two parts (Cynthiw), viz. a, the remoter portion 
of the blood-vessels, which ramifies near the periphery of the 
outer mantle, and 4, the portion of the capillary vessels which 
are distributed not far from the third fibrous layer of the 
mantle. If the blood is flowing in the former at a given 
moment in a direction from the heart, it is flowing at the same 
moment in the other in an opposite direction, or to the heart. 
The two portions of the capillary blood-vessels just mentioned 
are united by lateral branches. In correspondence with the 
regular change in the direction of pulsation in the heart-tube, 
the course of the blood changes in all the vessels described 
by me. 
The outer mantle of the Tunicata can be very easily sepa- 
rated from the inner one, and never coalesces with the epi- 
thelial cuticular layer of the latter, which, indeed, may be 
inferred & priord from the mode of its formation from the 
* Ann. Sci. Nat. tome y. p. 110, Lowig and Kolliker; Bronn, Weich- 
thiere, Abth. ii. Taf. 3. 
+ Quart. Journ. Micr. Sci. 1872, p. 288, pl. xii. fig. 9. 
